Alkali basalt from the Seifu Seamount in the Sea of Japan: post-spreading magmatism in a back-arc setting

Tomoaki Morishita,Hiroshi Sato,Shoji Arai,Tomoyuki Shibata,Akihiro Tamura,Masako Yoshikawa,Naoto Hirano,Hirochika Sumino,Rie Nauchi

doi:10.5194/se-11-23-2020

Abstract

Abstract. We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism.

Highlights

Numerous studies have investigated the magmatism that occurs during back-arc rifting (e.g., Martinez et al, 2001; Pearce and Stern, 2006)
The SSM basalt is characterized by LREE enrichment and no apparent anomalies in Eu and high-fieldstrength elements (HFSEs) relative to neighboring REE, whereas Rb and Th exhibit slight negative anomalies
Basalt samples were recovered from the Sea of Japan by the Ocean Drilling Program (ODP) Leg 127–128 cruises in the Yamato Basin (Sites 794 and 797) and Japan Basin (Site 795) (e.g., Tamaki et al, 1992) (Fig. 1). 40Ar/39Ar dating of these basalts indicated formation during back-arc magmatism at 21–18 Ma (Yamato Basin) and 25–15 Ma (Japan Basin) (Kaneoka et al, 1992)

Summary

Introduction

Numerous studies have investigated the magmatism that occurs during back-arc rifting (e.g., Martinez et al, 2001; Pearce and Stern, 2006). Geophysical data and sampling by the Ocean Drilling Program (ODP) have revealed the mechanisms of formation of the northern Sea of Japan (Tamaki et al, 1992) (Fig. 1). Lithospheric breakup and oceanic spreading began at the eastern margin of the continent at 32–28 Ma, and backarc magmatism ceased at 18–15 Ma (Tamaki et al, 1992). The basins in this region mainly comprise extended continental and oceanic crust (Tamaki et al, 1992).

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Discussion

Conclusion